Preparation of methylenimines



PREPARATION OF METHYLENIMINE Del., assignor to E. I.

John Lynde Anderson, Wilmington, D I

du Pont de Nemours and Company, Wilmington, a corporation of Delaware No Drawing. Application November 19, 1951, Serial No. 257,189

5 Claims. (Cl. 266--566) This invention relates to methylenimines and rnore particularly, to the preparation of those methylenrmrnes characterized by a methylene group attached to nitrogen which in turn is attached to a hydrogen-bearing carbon which may have up to two hydrocarbon radicals attached thereto. These compounds have the formula RiRaCHN=CH2 wherein R1 and R2 are hydrogen or alkyl radicals and may be alike or different.

Although at one time it was believed that formaldehyde reacted with primary amines to produce methylenimines of the type here considered, it was subsequently found this was in error. The products actually obtained by treatment or" various primary hydrocarbon amines with aqueous formaldehyde are the heXahydro-l,3,5-trihydrocarbon substituted-s-triazines. These compounds have the structure:

wherein R1 and R2 are hydrogen or hydrocarbon radicals.

So far as the prior art literature reveals, the methylenimines of the type herein considered have not been obtained heretofore in the rnonomolecular state.

An object of the present invention is to provide a process of preparing methylenimines of the formula R1R2CHN=CH2 wherein R1 and R2 are hydrogen or hydrocarbon radicals. A further object is to provide such a process whereby these compounds can be recovered in their monomolecular state. Other objects will be apparent from the description of the invention given hereinafter.

The above objects are accomplished according to the present invention by pyrolyzing a hexahydro-1,3,5-trihydrocarbon substituted-s-triazine at a temperature of at least 200 C. and isolating the resulting methylenimine substantially as formed. Generally, the pyrolysis will take place at a temperature above 300 C. and preferably at a temperature of 400 C. to 600 C.

Most of the methylenimines herein considered are extremely volatile and all of them are more volatile than the triazines from which they are derived, so their immediate removal from the reaction mixture presents no problem. However, to preserve these compounds in their monomolecular state they should be promptly condensed at low temperature and maintained in the liquid state as they readily polymerize to products which have more than the three units of the methylenimine found in the triazines from which they are prepared.

It has been discovered that by following the procedure "ice above the herein considered methylenimines which apparently have never been prepared in the monomolecular state before, may be readily and substantially quantitatively obtained in the monomolecular state from the corresponding triazine.

The following examples in which all proportions are by weight unless otherwise stated, illustrate specific em'- bodiments of the invention.

EXAMPLEl Preparation of N-methylmefl zylenimine V I A total of 10 parts of hexahydrol,3 ,5-trimethyl-striazine was added dropwise through a 6'? x rl."', bed'qof alumina-silica (Socony Vacuumbeads) at 425450 C., the system being maintained at l0- mm. of mercury.

-The vapors whichv resulted from thepyrolysis were led through a trap at 64 C. and caught and condensed in a trap at -l96 C. A quantitative conversion to N- methylrnethylenirnine was achieved. with virtually no residue remaining in the trap at -64 C. The product had a melting point of l19 C. and its vapor pressure equation obtained by plotting the log of its vapor pres-. sure against the reciprocal of the absolute temperature is given by the equation T(7.()5log p)='l.11 10 Determination of a low-temperature infrared spectrum showed that this compound possesses a very strong characterlstic absorption at 6.02,. N-methylmethylenimine on warming near its boiling pointv (ca. -35 C.) polymerized spontaneously to a. resinous or wax-like polymer not having the hexahydrotriazine structure.

When allowed to react at low temperaturev (ca. -80 C.) with activated cellulose,- an aminomethyl cellulose containing G.39% nitrogen was formed. J

When allowed to react at -80 C. with ketene a product formed which approximated a 1:1 adduct.

Reaction with methyl iodide at 80 C. in ether solution gave a quaternary compound in the ratio of 2 imines to 1 methyl iodide.

EXAMPLE 11 Preparation of N- ethylmethylenimine and condensed in a fourth trap at '196 C. The product which'was formed quantitatively was N-ethylmethylenimine which possessed a melting pointof '-132.5 C.

and a vapor pressure equation EXAMPLE HI Preparation 0 N-methylmethylcnimine As an alternate procedure to that of Example I, N- methylmethylenimine was obtained when hexa'hydrol,3,S-trimethyl-s-triazine was introduced in a stream of dry nitrogen onto quartz chipssheated at 5 30-5'48 C; at atmospheric pressure.

3 EXAMPLE IV Preparation of N-isopropylmethylenimine A total of parts of hexahydro-1,3,5-triisopropyl-striazine was introduced dropwise over a period of 45 minutes in a high-vacuum system maintained at 10 mm. over a bed of quartz chips at 525-540 C. The resultant vapors were passed through three traps at 50 C. and condensed in a fourth trap at 196 C. The melting point of the product was 99 C. Determination of the vapor pressure equation by plotting of the logarithm of the pressure against the reciprocal of the absolute temperature gave the following equation: T(7.60-log p)=1.45 X10 The imine polymerized spontaneously to a hard white solid on standing at temperatures within 50 C. below room temperature.

EXAMPLE V Preparation of N-isoamylmetlzylenimirze The procedure of Example IV was repeated except that 10 parts of hexahydro-l,3,S-tri-i-amyl-s-triazine was pyrolyzed at 530 C. over quartz chips in a high-vacuum system maintained at l0- mm. mercury, and gave a product which was collected at -l96 C. The product showed a melting point of about -l4l C. with glassing. This compound had a strong infrared absorption band at 602g. The absorption was determined at room temperature.

The above examples are merely illustrative and the invention broadly comprises treating by straight pyrolysis at a temperature of at least 200 C., these hexahydrotriazines to form methylenimines of the formula R1R2CI-IN=CH2 and isolating the methylenimines promptly to prevent their polymerization.

The invention is applicable generally to those hexahydrotriazines having the formula NCHRiRi where R1 and R2, alike or different, are hydrogen or hydrocarbon radicals. These hexahydrotriazines are characterized by an exocyclic hydrogen-bearing carbon attached to each ring nitrogen. This carbon may carry two other hydrogens as in the case of hexahydro-1,3,5- trimethyl-s-triazine or in place of one or both of such hydrogens this carbon may carry hydrocarbon radicals but, in all instances, it carries at least one hydrogen. The hydrocarbon radical attached to each ring nitrogen is suitably of from I to 8 carbons, inclusive, e. g., methyl, propyl, allyl, amyl, 'octyl, benzyl, and cyclohexyl. The preferred hexahydrotriazines are those where R1 and R2 in the above formula are either hydrogen or hydrocarbon radicals, the sum of the carbon in R1 and R2 not exceeding 7. Particularly preferred are those hexahydrotriazines where R1 and R2 in the formula are either hydrogen or alkyl radicals, the sum of the carbon in R1 and R2 not exceeding 4. The lower alkyl radicals having I to 4 carbons, inclusive, are particularly preferred. Specific hexahydrotriazines other than those of the examples, which are particularly suitable are hexahydro-1,3,5-trioctyl-s-triazine, hexahydro-l,3,5-tricyclohexyl-s-triazine, hexahydro-l,3,5-tributyl-s-triazine, and hexahydro-l,3,5- tri 2-butyl) amyl-s-triazine.

The pyrolysis should be carried out in apparatus that permits rapid heating of the hexahydrotriazine with subsequent cooling and isolation of the methylenimine. Suitable technique for carrying out the reaction consists in heating the triazine to a temperature of at least 200 C. for a short time followed by prompt isolation of the resultant methylenimine as formed and its condensation.

In carrying out the pyrolysis, the use of inert particulate material to transfer the heat facilitates the reaction and reduces the time and/ or temperature. Suitable transfer materials are those which contain the oxides of elements of atomic number of 13 to 15, aluminum, silicon, and phosphorus in particulate form, e. g.,. as beads.

A practical method of conducting the pyrolysis consists of introducing hexahydrotriazine in small quantities, such as in dropwise addition, to the heated pyrolysis chamber. The pyrolysis chamber is normally heated to above 300 C. and, preferably, from about 400 C. to 600 C., although higher temperatures, e. g., up to 800 C. or higher, particularly when no heat transfer bed is present, can be used effectively. The time of reaction is short, e. g., of the order of a few minutes at the longest, to obtain maximum yield.

Upon formation of the methylenirnine it should be isolated from the reaction mixture promptly and condensed at low temperature to prevent its polymerization. These methylenimines melt at temperatures of the order of l00 C. or lower. The use of an inert atmosphere or a high vacuum in the isolation and storage of the methylenimine is desirable. Using the process of this invention there may be readily prepared methylenimines of the formula R1R2CHN=CH2 wherein R1 and R2 are hydrogen or hydrocarbon radicals of up to 7 carbon atoms, the sum of the carbons in R1 and R2 not exceeding 7, which are the preferred products. Examples of these methylenimines include N-benzylmethylenimine, N isooctylmethylenimine, N cyclohexylmethylenimine, N butylmethylenimine, and N (2 butylamyl)methylenimine.

As indicated in the examples, these methylenimines react with themselves at relatively low temperatures, e. g., room temperature, to give polymers which contain more than three units of the monomer and which differ in properties from the triazine. Further, these methylenimines react with quaternary-forming materials, ketene, and various other reagents.

An advantage of this invention is that it provides a readily carried out process of preparing the herein considered methylenimines which have never been known in their monomolecular state heretofore. A further advantage is that this process may be carried out economically and without appreciable loss otherwise of the hexahydrotriazine used as the source material.

As many apparently widely different embodiments of this invention may be made without departing from the spirit and scope thereof, it is to be understood that the invention is not limited to the specific embodiments thereof except as defined in the appended claims.

The invention claimed is:

1. Process of preparing a methylenimine of the formula R1R2CHN=CH2 in which R1 and R2 are from the group consisting of hydrogen and hydrocarbon radicals of l to 7 carbons, inclusive, the sum of the carbons in R1 and R1 not exceeding 7, said process comprising pyrolyzing a heXahydro-trihydrocarbon substituted-s-triazine wherein each ring nitrogen is attached to an exocyclic substituent of the formula R1R2HC- in which R1 and R2 are from the group consisting of hydrogen and hydrocarbon radicals of 1 to 7 carbons, inclusive, the

sum of the carbons in R1 and R2 not exceeding 7, at a temperature of at least 300 C. and isolating the resulting methylenimine substantially as formed.

2. Process as set forth in claim 1 wherein said triazine is a hexahydro-trialkyl substituted-s-triazine.

3. Process as set forth in claim 2 wherein said pyrolysis is carried out at a temperature of 400 C. to 600 C.

4. Process of preparing a methylenimine of the formula R1R2CHN=CHz in which R1 and R2 are from the group consisting of hydrogen and hydrocarbon radi- CHR Rz 1 C: CHz RiRzflcl T NCHRrRn wherein R1 and R2 are from the group consisting of hydrogen and hydrocarbon radicals of 1 to 7 carbons, inclusive, the sum of the carbons in R1 and R2 not exceeding 7, at a temperature of at least 300 C. and isolating the resulting methylenimine substantially as formed.

5. Process as set forth in claim 4 wherein said pyrolysis is carried out at a temperature of 400 C. to 600 C.

References Cited in the file of this patent Graymore: J. Chem. Soc. (London), part 11, 1947, page 1117. 

1. PROCESS OF PREPARING A METHYLENIMINE OF THE FORMULA R1R2CHN=CH2 IN WHICH R1 AND R2 ARE FROM THE GROUP CONSISTING OF HYDROGEN AND HYDROCARBON RADICALS OF 1 TO 7 CARBONS, INCLUSIVE, THE SUM OF THE CARBONS IN R1 AND R2 NOT EXCEEDING 7, SAID PROCESS COMPRISING PYROLYZING A HEXAHYDRO-TRIHYDROCARBON SUBSTITUTED-S-TRIAZINE WHEREIN EACH RING NITROGEN IS ATTACHED TO AN EXOCYCLIC SUBSTITUENT OF THE FORMULA R1R2HC- IN WHICH R1 AND R2 ARE FROM THE GROUP CONSISTING OF HYDROGEN AND HYDROCARBON RADICALS OF 1 TO 7 CARBONS, INCLUSIVE, THE SUM OF THE CARBONS IN R1 AND R2 NOT EXCEEDING 7, AT A TEMPERATURE OF AT LEAST 300* C. AND ISOLATING THE RESULTING METHYLENIMINE SUBSTANTIALLY AS FORMED. 